He’s been among the researchers looking over the files and has found they contain about 20 different Windows-based exploits—four of which appear to leverage previously unknown software vulnerabilities.

Each exploit works as a program that takes advantage of a security flaw. Researchers are still examining the leaked files, but the exploits appear to work on older Windows versions including NT, XP and Windows 7.

However, computers running Windows Server are particularly at risk, Hickey said. That’s because the exploits are generally designed to leverage vulnerabilities in a machine’s online server functions.

Hickey found that one such exploit included in the leak, called Eternalblue, can remotely cause older versions of Windows to execute code. In a video , he demonstrated this against a machine running Windows Server 2008 R2 SP1 and pulled off the hack in less than two minutes.

“An attacker can use these tools to effectively hack into Windows computers around the world and run their own code for attacks,” he said.

For example, a hacker could open a backdoor into the machine to upload other malicious that can act as ransomware or steal sensitive data.

What makes the exploits a particularly big problem is that older versions of Windows Server remain widely used. The latest version, Windows Server 2016, was launched only last year.

“These are very serious vulnerabilities, with a very serious impact on Microsoft,” Hickey said.

Microsoft has yet to release a patch, and it’s unclear when that might happen. On Friday, the software giant said it was still studying the leaked exploits.

Computers that are behind a firewall should be safe. For those that aren’t, companies should consider disabling certain functions that the exploits use, said Amol Sarwate, director of engineering for security firm Qualys.

For example, the Eternalblue exploit leverages the Server Message Block and NetBT protocols to hijack the system.

He also recommends that companies take inventory of their IT assets so they know which servers might be vulnerable.

“Customers should proactively keep an eye out for this, and of course have a strategy to patch them” when a patch becomes available, Sarwate said. “Asset management is very crucial here.”